diff --git a/src/notion-to-md.ts b/src/notion-to-md.ts
index d83948a..a3be904 100644
--- a/src/notion-to-md.ts
+++ b/src/notion-to-md.ts
@@ -23,7 +23,7 @@ export class NotionToMarkdown {
   private notionClient: Client;
   private config: ConfigurationOptions;
   private targetPage: string;
-  private customTransformers: Record<string, CustomTransformer>;
+  private customTransformers: Record<BlockType, CustomTransformer>;
 
   constructor(options: NotionToMarkdownOptions) {
     this.notionClient = options.notionClient;
diff --git a/src/types/index.ts b/src/types/index.ts
index c95bd96..d85275a 100644
--- a/src/types/index.ts
+++ b/src/types/index.ts
@@ -142,4 +142,4 @@ export type CalloutIcon =
 
 export type CustomTransformer = (
   block: ListBlockChildrenResponseResult
-) => BlockType | Promise<string>;
+) => string | Promise<string>;